(a) Field of the Invention
The present invention relates to a bolt-nut assembly which includes a bolt, a nut and a washer and has a self-locking function that prevents the nut from becoming loose from the bolt.
(b) Description of the Related Art
Typically, bolt-nut assemblies are widely used, in which a bolt is inserted through elements and a nut is then tightened on the bolt to fasten the elements to each other, in order to couple construction materials or mechanical elements to each other.
However, with the passage of time, such bolt-nut assemblies may gradually become loose because of external vibrations or shocks, thus deteriorating the reliability of the coupling portion.
In an effort to prevent a nut from becoming loose by reverse rotation of the nut, a variety of auxiliary fastening means have been proposed. A representative example of the auxiliary fastening means was disclosed in Korean Utility Model Registration No. 0241791 (entitled “Bolt with safe-locking nut”).
In Korean Utility Model Registration No. 0241791, as shown in FIG. 1, a locking groove 1-3 is formed in a threaded portion of a bolt body 1. A stopper 2-2 provided on a washer is locked into the locking groove 1-3. One-way stop protrusions 2-3 and 3-3 are respectively provided on the washer 2 and a nut 3 on the junction between the washer 2 and the nut 3.
This conventional technique has, however, the following problems.
1) The nut 3 and the washer 2 interact with each other in such a way that the washer 2 and the nut 3 pass over each other with compression strain therebetween rather than having the washer 2 elastically bend with respect to the nut 3 and then return to its original state. Thus, if the washer is made of a hard material, the one-way stop protrusions 3-2 of the nut 3 may not be able to pass over the one-way stop protrusions 2-3 of the washer 2.
Meanwhile, if the washer is made of soft material (for example, rubber, plastic, etc.), it cannot withstand a high bolt coupling torque and compression force. Furthermore, it cannot withstand a severe environment, for example, high-temperature and high-pressure environment, for a long period of time. Hence, the conventional bolt-nut structure cannot be used in a fastening element for a structure or power apparatus which is exposed to vibrations, shocks, high temperature conditions, etc.
Particularly, in the case where the washer is made of soft material, when the nut is tightened, a surface of the washer that comes into contact with the nut is compressed by the nut in the same shape (hexagonal shape) as that of the nut. The compressed surface of the washer functions to resist the force that would act to loosen the nut. Thus, the one-way stop protrusions 2-3 and 3-3 which are formed on the washer 2 and the nut 3 become useless.
2) If the nut 3 is forcibly tightened onto the bolt body 1, the one-way stop protrusions 2-3 and 3-3 which are formed on the washer 2 and the nut 3 may be worn or damaged (broken), thus malfunctioning.
3) Even if the nut 3 has been forcibly tightened on the bolt body 1, the one-way stop protrusions 2-3 and 3-2 of the washer 2 and the nut 3 are not completely brought into contact with each other or do not reliably lock into each other, thus causing the fastening to be unsatisfactory.
4) Therefore, after the nut 3 has been tightened onto the bolt body 1, the nut 3 is easily loosened despite comparatively low vibration, thereby making it difficult to keep a prescribed torque.
5) Furthermore, when tightening the nut 3 on the bolt body 1, compression force between the one-way stop protrusions 2-3 and 3-2 of the washer 2 and the nut 3 largely varies while the one-way stop protrusions 2-3 and 3-2 pass over each other. Thereby the tightening torque cannot be maintained constant. That is, during the operation of tightening the nut 3 on the bolt body 1, the following processes are repeated; while the one-way stop protrusions 3-2 of the nut 3 slide along the one-way stop protrusions 2-3 of the washer 2, the tightening torque gradually increases and then reaches a peak, and the moment after the one-way stop protrusions 3-2 completely pass over the one-way stop protrusions 2-3, the tightening torque is rapidly reduced. Hence, the tightening torque is not constant.
6) In addition, once the nut 3 has been tightened on the bolt body 1, it is impossible to separate the nut 3 from the bolt to reuse it, thus wasting material. The reason for this is that if the nut 3 is forcibly turned in the direction in which it is loosened, the one-way stop protrusions 2-3 and 3-2 of the washer 2 and the nut 3 which have engaged with each other are damaged or broken (at least the edges thereof are severely worn so that they cannot be reused).
7) When washers are manufactured, the amount of material required to form each washer is increased, and the shape of the washer is complex. Thus, the production cost per unit is markedly increased.
8) The one-way stop protrusions 2-3 and 3-2 may be worn or damaged for various reasons, thus causing the nut 3 to become loose. After this, there is no longer any means for preventing the nut from becoming loose.